You right, Sumatrans were Buddhist but they were not ancestors of present Indonesian Buddhist. Buddhism in Nusantara (old Indonesia) was over when Islam came. Present Indonesian Buddhist are Chineses who came as emigrant.
How about Malaysian Buddhists ? are they Chineses or Thais and Cambodian or Malays ?
There are four main sects of Buddhism here...
1) Mahayana - most of Buddhists in Malaysia are Mahayanists, a great majority of them are Chinese, some are Vietnamese, Koreans and Japanese. Mahayana Buddhism was introduced by Chinese immigrants...
2) Theravada - it's the oldest sect in Malaysia, many of the Malays' ancestors were Theravadists... I dont know about other part of Malaysia.. but the northern malaysian malays were actually Thais, when they embraced Islam massively in the 15th century... the remaining Buddhists are still continue living as ethnic Thais until today. During British colonial time, many Theravadists migrated from Sri Lanka (ethnic Sinhalese), India, Myanmar (ethnic Burmese) and Thailand. Recently there are thousands of Theravadist immigrants from Cambodia and Laos too. Majority of the Theravadist are Thais, with a high % from Sinhalese and Burmese too...
3) Tantrayana (Tibetan) Buddhism - very new to Malaysia, most of the Tantrayanists are recent converts from ethnic Chinese.
4) Soka Gakkai Buddhism - a new Buddhism sect from Japan... many of the converts here are ethnic Chinese... with some Japanese.
The only Malay (well she's a half Malay, half Chinese) that I've heard of was Nyonya Binti Tahir who was a Buddhist after the Shariah Court declared her as a Buddhist as she was adopted by a Buddhist family and never practice Islam.